The epithelial components of the mammary gland arise from stem cells that have the capacity for self- renewal as well as multi-lineage differentiation. Recent studies have provided support for the cancer stem cell hypothesis, which holds that these cells or their immediate progeny may be targets for mammary carcinogenesis which results from disregulation of stem cell self-renewal pathways. This results in expanded stem cell populations which are the targets for further carcinogenic events. Resulting tumors consist of heterogeneous cell populations driven by a stem cell component that maintains the ability to self-renew as well as to differentiate into cells that comprise the bulk of the tumor. We have developed techniques for the isolation and characterization of stem cells from both human mammary glands and mammary tumors. Furthermore, we have demonstrated that stem cell markers such as aldehyde hydrogenase 1 may be utilized to detect mammary stem cells in situ in formalin fixed paraffin embedded tissue. We propose to utilize these new tools to directly test a key component of the cancer stem cell hypothesis regarding the cellular origin and early molecular events which leads to human breast carcinogenesis. We propose that a common biological link between hereditary and sporadic breast cancer is that both may be initiated by disregulation of mammary stem cell self-renewal pathways. In hereditary breast cancer, this may involve deletion of BRCA-1, whereas in sporadic breast cancers, disregulation of genes such as HER-2 and PTEN result in clonal stem cell expansion providing targets for secondary transforming events. In order to test these hypotheses, we propose to utilize both in vitro systems and a humanized mouse xenograft model, as well as in situ approaches developed by our laboratory to elucidate the roles of BRCA-1, HER-2 and PTEN in the regulation of mammary stem cell self-renewal and carcinogenesis. These studies may lead to new approaches for the early detection and prevention of breast cancer. PUBLIC HEALTH RELEVANCE: Breast cancer is the most common malignancy in women and the 2nd leading cause of cancer death. Since breast cancer may originate in normal breast stem cells, understanding the earliest changes in these cells during cancer formation is critical for the development of novel strategies to detect and prevent this cancer.